Interruption in a vacuum circuit breaker is achieved by vacuum interrupters which require only a short contact gap for circuit interruption. Vacuum interrupters typically comprise an insulating cylindrical envelope composed, for example, of glass or alumina, with endcaps mounted at each end to form a closed cylinder. Stationary and movable current conducting terminals are mounted through the endcaps at opposite ends of the cylindrical envelope. The movable terminal is moved axially to make or break contact between the movable and stationary contacts, thereby making or breaking the electrical circuit.
Vacuum interrupters of this type are required to maintain an internal vacuum on the order of about 10.sup.-7 Torr to interrupt the current flowing in the electrical circuit. Loss of vacuum permits air or other molecules to enter the evacuated volume, which reduces the internal vacuum and thereby reduces the interrupting capacity and dielectric strength of the internal vacuum. It is essential, therefore, that the interior volume of the interrupter remains hermetically sealed from the external atmosphere to maintain the internal vacuum. The quality of the components and materials must be consistently high to prevent introduction of contaminants into the internal vacuum, and to prevent the external walls of the interrupter from developing weak or porous areas which would threaten the integrity of the internal vacuum.
Suitable component materials have been developed and in general, the interrupter components including the insulating cylindrical envelope, the current conducting terminals, and the endcaps are not prone to degradation which would threaten the integrity of the internal vacuum. Problems may arise, however, in assembly of the components to form a hermetically sealed cylinder. Assembly of the vacuum interrupter components requires at least one brazing seam to hermetically seal the insulating envelope to the endcaps, and another brazing seam to hermetically seal the endcaps to the current carrying terminals.
FIG. 1 illustrates conventional prior art vacuum interrupter 10 comprising cylindrical insulating envelope 12 composed of alumina or the like, endcaps 14 and 16 mounted at opposite ends of insulating envelope 12, with stationary terminal 18 and movable terminal 20 mounted in opposite endcaps. According to prior art practices, annular connector 22 is mounted between the insulating envelope and each endcap and is brazed to the endcaps to hermetically seal the envelope and the endcaps at brazed joint 24. Annular connector 22 generally comprises a material having a coefficient of thermal expansion corresponding to that of the cylindrical insulating envelope.
When the insulating envelope comprises alumina or the like, as shown, metallized end surface 26 is provided at each circumferential edge of the insulating envelope, and the annular connector is additionally brazed to the insulating envelope at brazed joint 28. If the insulating envelope comprises a glass material, one edge of annular connector 22 is typically embedded in the glass envelope, and a brazing seam is required only at the interface between the annular ring and the endcap.
To provide brazed seams having sufficient strength and reliability using conventional brazing techniques, it is important that the materials being brazed have substantially similar coefficients of thermal expansion. The annular connector conventionally used to seal the insulating envelope and the endcaps, as described above, must seal hermetically with materials having different characteristics, namely the insulating envelope and the metallic endcaps. Since the annular connector typically comprises a material having a coefficient of thermal expansion matching that of the insulating envelope, its coefficient of thermal expansion does not match that of the endcap, and consequently, the brazing seam joining the annular connector and the endcap may be prone to fatigue, deterioration and distortion during vacuum operation of the interrupter and/or during the brazing process.
As described above, conventional vacuum interrupters require at least one brazing seam 24 extending a length corresponding to the circumference of the insulating envelope to hermetically seal the envelope to each endcap. When the insulating envelope comprises alumina or the like, conventional vacuum interrupters require two brazing seams 24 and 28, each extending a length corresponding to the circumference of the insulating envelope. The brazing seams represent a critical area of control, since any inconsistency in the brazing technique or deterioration of the brazing materials may leave leakage paths in the brazed seams which permit introduction of contaminants and destroy the integrity of the internal vacuum. One strategy to maintain the integrity of the internal vacuum may thus involve reducing the total length of brazed seams required to assemble and hermetically seal the vacuum interrupter.
The stationary and movable current carrying terminals are mounted through a central portion of endcaps mounted at opposite ends of the insulating envelope. The endcaps must be rigid enough to prevent any diaphramming or bending due to normal contact pressure during reciprocation of the movable contact or electromagnetic forces which are applied during current interruption. Consequently, conventional endcaps comprise a thick section of a rigid, relatively heavy metallic material. The application of forces typically generated during vacuum interruption to an endcap which is not sufficiently rigid results in axial and/or radial movement of the stationary terminal, which destroys or substantially reduces the interrupting capability of the vacuum interrupter.
Accordingly, it is an objective of the present invention to provide a vacuum interrupter having an internal volume which is hermetically sealed from the external atmosphere in a manner which substantially reduces contamination and leakage of the internal vacuum.
It is another objective of the present invention to provide an improved vacuum interrupter which is durable and reliable over the course of longterm operations.
It is still another objective of the present invention to provide a vacuum interrupter having a reduced total length of brazed seams to improve internal vacuum maintenance during longterm operation.
It is yet another objective of the present invention to provide an improved stationary terminal assembly for use in a vacuum interrupter which demonstrates increased structural rigidity.